With the recent improvement in performance of observation devices installed in an aircraft and an artificial satellite, it is required to increase the transmission capacity in transmitting data from an aircraft or an artificial satellite to the earth. In order to respond to an anticipated request to further increase the transmission capacity, research and development on a free space optical communication system have been carried out by which the transmission capacity can be increased dramatically compared to the conventionally used microwave wireless communication system.
The current free space optical communication system uses the intensity modulation and direct detection scheme in which communication is performed using light intensity making “1” and “0” of digital signals to be transmitted correspond to “ON” and “OFF” of optical signals, or the pulse position modulation scheme. This makes it difficult to increase the transmission capacity further. A further increase in transmission capacity requires applying digital coherent technologies used in the optical fiber communication system to the free space optical communication system.
In the free space optical communication system, a laser beam with a wavelength (1.55 micrometers (μm)) employed in the optical fiber communication system is mainly used, and it enables communication to propagate the laser beam through the atmosphere. As a result, the free space optical communication is easily influenced by the state of the atmosphere, and it becomes unstable easily compared to the microwave communication. A fluctuation of the laser beam due to the state of the atmosphere is generally called an atmospheric fluctuation. The atmospheric fluctuation becomes a major problem especially when the received laser beam is coupled to a single-mode fiber with a small core diameter that is used in digital coherent technologies.
Patent Literature 1 discloses technologies to solve such a problem. A free space optical receiver disclosed in Patent Literature 1 includes light collecting means for collecting laser light having propagated through a free space transmission path, a multimode transmission medium, mode converting means, mode separating means, a plurality of single mode transmission media, a plurality of light receiving means, and signal processing means.
Signal light subjected to a wave-front fluctuation after having propagated through a free space transmission path is subjected to a mode conversion in the mode conversion means, and then separated into a plurality of orthogonal modes by the mode separating means. In this case, the mode converting means makes it possible to receive selectively only single mode signal light that can be coupled to the single mode fiber and can be separated in the mode separating means from among many propagation modes of the multimode transmission medium. As a result, according to the free space optical receiver described in Patent Literature 1, it is possible to satisfy both of a higher bit rate of the transmission signal and highly efficient fiber coupling, and it becomes possible to achieve the high-capacity free space optical communication.
Patent Literature 2 discloses an optical transmission system in which identical optical signals are transmitted in parallel using optical transmission lines such as multicore fibers. A redundant optical signal receiver included in the optical transmission system is composed of an optical signal receiver and a digital signal processor. The optical signal receiver performs a coherent detection on optical signals transmitted through the optical transmission lines, performs an analog-to-digital conversion on the resultant signals, and then outputs digital signals. The digital signal processor receives the input of the digital signals output from the optical signal receiver, performs phase synchronization and waveform equalization, and synthesizes signals. The digital signal processor is composed of an adaptive equalizer, a phase synchronizer, and a signal synthesizer; and a phase difference and a phase synchronization signal outputted from the phase synchronizer, and a phase synchronization signal outputted from a phase synchronizer used as a reference are fed back to the adaptive equalizer, respectively. The signal synthesizer receives input of the phase synchronization signal outputted from the phase synchronizer, performs equal gain combining or maximal ratio combining, and outputs a synthesized signal.
With such a configuration, the optical transmission system disclosed in Patent Literature 2 can facilitate high-capacity transmission with improved transmission quality.